Retroreflective cube corner sheeting mold and method for making the same

ABSTRACT

A method is disclosed for manufacturing a plurality of laminae for use in a mold suitable for use in forming retroreflective cube corner articles. Each lamina has opposing first and second major surfaces defining therebetween a first reference plane. Each lamina further includes a working surface connecting the first and second major surfaces. The working surface defines a second reference plane substantially parallel to the working surface and perpendicular to the first reference plane and a third reference plane perpendicular to the first reference plane and the second reference plane. The method includes orienting a plurality of laminae to have their respective first reference planes parallel to each other and disposed at a first angle relative to a fixed reference axis. At least two groove sets are formed in the working surface. Each groove set includes at least two parallel adjacent V-shaped grooves in the working surface of the laminae. The at least two groove sets form first, second and third groove surfaces that intersect substantially orthogonally to form a plurality of cube corner elements. Each of the plurality of cube corner elements is preferably located on essentially one of the plurality of lamina. The plurality of laminae can be oriented at a second angle relative to the fixed reference axis prior to forming at least one of the groove sets. A mold according to the present invention and a retroreflective article made therefrom are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of pending U.S. patent application Ser.No. 08/887,074, filed Jul. 2, 1997.

FIELD OF THE INVENTION

[0002] The present invention relates generally to molds suitable for usein forming cube corner retroreflective sheeting, to methods for makingthe same, and to retroreflective sheeting formed from such molds. Inparticular, the invention relates to molds formed from a plurality ofthin laminae and to methods for making the same.

BACKGROUND OF THE INVENTION

[0003] Retroreflective materials are characterized by the ability toredirect light incident on the material back toward the originatinglight source. This property has led to the wide-spread use ofretroreflective sheeting in a variety of conspicuity applications.Retroreflective sheeting is frequently applied to flat, rigid articlessuch as, for example, road signs and barricades; however, it is alsoused on irregular or flexible surfaces. For example, retroreflectivesheeting can be adhered to the side of a truck trailer, which requiresthe sheeting to pass over corrugations and protruding rivets, or thesheeting can be adhered to a flexible body portion such as a roadworker's safety vest or other such safety garment. In situations wherethe underlying surface is irregular or flexible, the retroreflectivesheeting desirably possesses the ability to conform to the underlyingsurface without sacrificing retroreflective performance. Additionally,retroreflective sheeting is frequently packaged and shipped in rollform, thus requiring the sheeting to be sufficiently flexible to berolled around a core.

[0004] Two known types of retroreflective sheeting are microsphere-basedsheeting and cube corner sheeting. Microsphere-based sheeting, sometimesreferred to as “beaded” sheeting, employs a multitude of microspherestypically at least partially embedded in a binder layer and havingassociated specular or diffuse reflecting materials (e.g., pigmentparticles, metal flakes or vapor coats, etc.) to retroreflect incidentlight. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178(McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No.5,066,098 (Kult). Advantageously, microsphere-based sheeting cangenerally be adhered to corrugated or flexible surfaces. Also, due tothe symmetry of beaded retroreflectors, microsphere based sheetingexhibits a relatively orientationally uniform total light return whenrotated about an axis normal to the surface of the sheeting. Thus, suchmicrosphere-based sheeting has a relatively low sensitivity to theorientation at which the sheeting is placed on a surface. In general,however, such sheeting has a lower retroreflective efficiency than cubecorner sheeting.

[0005] Cube corner retroreflective sheeting comprises a body portiontypically having a substantially planar base surface and a structuredsurface comprising a plurality of cube corner elements opposite the basesurface. Each cube-corner element comprises three mutually substantiallyperpendicular optical faces that intersect at a single reference point,or apex. The base of the cube corner element acts as an aperture throughwhich light is transmitted into the cube corner element. In use, lightincident on the base surface of the sheeting is refracted at the basesurface of the sheeting, transmitted through the bases of the cubecorner elements disposed on the sheeting, reflected from each of thethree perpendicular cube-corner optical faces, and redirected toward thelight source. The symmetry axis, also called the optical axis, of a cubecorner element is the axis that extends through the cube corner apex andforms an equal angle with the three optical faces of the cube cornerelement. Cube corner elements typically exhibit the highest opticalefficiency in response to light incident on the base of the elementroughly along the optical axis. The amount of light retroreflected by acube corner retroreflector drops as the incidence angle deviates fromthe optical axis.

[0006] The maximum retroreflective efficiency of cube cornerretroreflective sheeting is a function of the geometry of the cubecorner elements on the structured surface of the sheeting. The terms‘active area’ and ‘effective aperture’ are used in the cube corner artsto characterize the portion of a cube corner element that retroreflectslight incident on the base of the element. A detailed teaching regardingthe determination of the active aperture for a cube corner elementdesign is beyond the scope of the present disclosure. One procedure fordetermining the effective aperture of a cube corner geometry ispresented in Eckhardt, Applied Optics, v. 10, n. Jul. 7, 1971 pp.1559-1566. U.S. Pat. No. 835,648 to Straubel also discusses the conceptof effective aperture. At a given incidence angle, the active area canbe determined by the topological intersection of the projection of thethree cube corner faces onto a plane normal to the refracted incidentlight with the projection of the image surfaces for the thirdreflections onto the same plane. The term ‘percent active area’ is thendefined as the active area divided by the total area of the projectionof the cube corner faces. The retroreflective efficiency ofretroreflective sheeting correlates directly to the percentage activearea of the cube corner elements on the sheeting.

[0007] Predicted total light return (TLR) for a cube corner matched pairarray can be calculated from a knowledge of percent active area and rayintensity. Ray intensity may be reduced by front surface losses and byreflection from each of the three cube corner surfaces for aretroreflected ray. Total light return is defined as the product ofpercent active area and ray intensity, or a percentage of the totalincident light which is retroreflected. A discussion of total lightreturn for directly machined cube corner arrays is presented in U.S.Pat. No. 3,712,706 (Stamm).

[0008] Additionally, the optical characteristics of the retroreflectionpattern of retroreflective sheeting are, in part, a function of thegeometry of the cube corner elements. Thus, distortions in the geometryof the cube corner elements can cause corresponding distortions in theoptical characteristics of the sheeting. To inhibit undesirable physicaldeformation, cube corner elements of retroreflective sheeting aretypically made from a material having a relatively high elastic modulussufficient to inhibit the physical distortion of the cube cornerelements during flexing or elastomeric stretching of the sheeting. Asdiscussed above, it is frequently desirable that retroreflectivesheeting be sufficiently flexible to allow the sheeting to be adhered toa substrate that is corrugated or that is itself flexible, or to allowthe retroreflective sheeting to be wound into a roll for storage andshipping.

[0009] Cube corner retroreflective sheeting is manufactured by firstmanufacturing a master mold that includes an image, either negative orpositive, of a desired cube corner element geometry. The mold can bereplicated using nickel electroplating, chemical vapor deposition orphysical vapor deposition to produce tooling for forming cube cornerretroreflective sheeting. U.S. Pat. No. 5,156,863 to Pricone, et al.provides an illustrative overview of a process for forming tooling usedin the manufacture of cube corner retroreflective sheeting. Knownmethods for manufacturing the master mold include pin-bundlingtechniques, direct machining techniques, and laminate techniques. Eachof these techniques has benefits and limitations.

[0010] In pin bundling techniques, a plurality of pins, each having ageometric shape on one end, are assembled together to form a cube-cornerretroreflective surface. U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat.No. 3,926,402 (Heenan), U.S. Pat. No. 3,541,606 (Heenan et al.), andU.S. Pat. No. 3,632,695 to Howell provide illustrative examples. Pinbundling techniques offer the ability to manufacture a wide variety ofcube corner geometries in a single mold. However, pin bundlingtechniques are economically and technically impractical for making smallcube corner elements (e.g. less than about 1.0 millimeters).

[0011] In direct machining techniques, a series of grooves are formed ina unitary substrate to form a cube-corner retroreflective surface. U.S.Pat. No. 3,712,706 to Stamm and U.S. Pat. No. 4,588,258 to Hoopmanprovide illustrative examples. Direct machining techniques offer theability to accurately machine very small cube corner elements which arecompatible with flexible retroreflective sheeting. However, it is notpresently possible to produce certain cube corner geometries that havevery high effective apertures at low entrance angles using directmachining techniques. By way of example, the maximum theoretical totallight return of the cube corner element geometry depicted in U.S. Pat.No. 3,712,706 is approximately 67%.

[0012] In laminate techniques, a plurality of laminae, each laminahaving geometric shapes on one end, are assembled to form a cube-cornerretroreflective surface. German Provisional Publication (OS) 19 17 292,International Publication Nos. WO 94/18581 (Bohn, et al.), WO 97/04939(Mimura et al.), and WO 97/04940 (Mimura et al.), each disclose a moldedreflector wherein a grooved surface is formed on a plurality of plates.The plates are then tilted by a certain angle and each second plate isshifted crosswise. This process results in a plurality of cube cornerelements, each element formed by two machined surfaces on a first plateand one side surface on a second plate. German Patent DE 42 36 799 toGubela discloses a method for producing a molding tool with a cubicalsurface for the production of cube comers. An oblique surface is groundor cut in a first direction over the entire length of one edge of aband. A plurality of notches are then formed in a second direction toform cube corner reflectors on the band. Finally, a plurality of notchesare formed vertically in the sides of the band. German ProvisionalPatent 44 10 994 C2 to Gubela is a related patent. The reflectorsdisclosed in Patent 44 10 994 C2 are characterized by the reflectingsurfaces having concave curvature.

BRIEF SUMMARY OF THE INVENTION

[0013] The present invention relates to a master mold suitable for usein forming retroreflective sheeting from a plurality of laminae andmethods of making the same. Advantageously, master molds manufacturedaccording to methods disclosed herein enable the manufacture ofretroreflective cube corner sheeting that exhibits retroreflectiveefficiency levels approaching 100%. To facilitate the manufacture offlexible retroreflective sheeting, the disclosed methods enable themanufacture of cube corner retroreflective elements having a width assmall as 0.010 millimeters. Additionally, the present applicationenables the manufacture of a cube corner retroreflective sheeting thatexhibits symmetrical retroreflective performance in at least twodifferent orientations. Efficient, cost-effective methods of makingmolds formed from a plurality of laminae are also disclosed.

[0014] A plurality of laminae are machined simultaneously to form aplurality of cube corner elements. The three mutually perpendicularoptical faces of each cube corner element are preferably formed on oneof the plurality of laminae. That is, individual or discrete cube cornerelements preferably do not extend across more than one lamina. All threeoptical faces are preferably formed by the machining process to ensureoptical quality surfaces. A planar interface is preferably maintainedbetween adjacent laminae during the machining phase and subsequentthereto so as to minimize alignment problems and damage due to handlingof the laminae.

[0015] A plurality of laminae are manufactured for use in a moldsuitable for use in forming retroreflective cube corner articles. Eachlamina has opposing first and second major surfaces definingtherebetween a first reference plane. Each lamina further includes aworking surface connecting the first and second major surfaces. Theworking surface defines a second reference plane substantially parallelto the working surface and perpendicular to the first reference planeand a third reference plane perpendicular to the first reference planeand the second reference plane. The method includes orienting aplurality of laminae to have their respective first reference planesparallel to each other and disposed at a first angle relative to a fixedreference axis. At least two groove sets are formed in the workingsurface. Each groove set includes at least two parallel adjacentV-shaped grooves in the working surface of the laminae. The at least twogroove sets form first, second and third groove surfaces that intersectsubstantially orthogonally to form a plurality of cube corner elements.Each cube corner element is preferably located on essentially one of theplurality of lamina. The plurality of laminae can be oriented at asecond angle relative to the fixed reference axis prior to forming atleast one of the groove sets.

[0016] In one embodiment, the step of forming at least two groove setsincludes forming a first groove set including at least two paralleladjacent V-shaped grooves in the working surface of each of the laminae.Each of the adjacent grooves defines a first groove surface and a secondgroove surface that intersect substantially orthogonally to form a firstreference edge on each of the respective laminae. A second groove set isformed including at least one groove in the working surfaces of theplurality of laminae. Each groove in the second groove set defines athird groove surface that intersects substantially orthogonally with thefirst and second groove surfaces to form at least one first cube cornerelement located on essentially a single lamina.

[0017] The first cube corner element preferably comprises a plurality ofcube corner elements. Each of the plurality of cube corner elements arelocated on essentially one lamina. An interface between adjacent firstand second major surfaces is preferably planar. Each lamina measuresbetween about 0.025 millimeters and about 1.0 millimeters in thickness,and more preferably from about 0.1 to about 0.6 millimeters.

[0018] The method includes the step of orienting the plurality oflaminae including assembling the laminae in a fixture defining a baseplane. The first angle measures between about 5° and about 85° from afixed reference axis normal to the base plane, and more preferablybetween about 10° and about 65° and most preferably about 25° to about45°.

[0019] The step of forming the groove sets comprises forming at leastone of the groove sets parallel to the base plane defined by thefixture. Alternatively, the groove sets can be formed at an acute anglerelative to the base plane defined by the fixture. The groove sets canalso be formed to vary the distance between adjacent grooves atdifferent depths in the working surface of the laminae.

[0020] The process of forming the groove sets can comprise removingportions of each of the plurality of lamina proximate the workingsurface of the plurality of laminae using a material removal technique.The first, second and third groove surfaces are formed essentially fromthe material removal technique. The groove sets can be formed byinducing relative motion between the plurality of laminae and a cuttingtool. The step of forming the groove sets comprises a machiningoperation selected from the group of machining operations consisting ofruling, fly-cutting, grinding, and milling. The grooves preferably havean included angle that measures between about 10° and about 170°.

[0021] In one embodiment, the plurality of lamina can be oriented tohave their respective first reference planes parallel to each other anddisposed at a second angle relative to the fixed reference axis prior toforming the second groove set. The step of orienting the plurality oflaminae to have their respective first reference planes parallel to eachother and disposed at a second angle relative to the fixed referenceaxis comprises re-assembling the plurality of lamina in a suitablefixture. In one embodiment, the step of orienting the plurality oflaminae to have their respective first reference planes parallel to eachother and disposed at a second angle relative to the fixed referenceaxis comprises rotating a plurality of the laminae 180° about an axisperpendicular to the second reference plane.

[0022] The cube corner elements are typically arranged in opposingpairs. In an alternate embodiment, optical axes of the cube cornerelements can be generally parallel to provide an asymmetrical totallight return about a 360° range of orientation angles.

[0023] Also disclosed is a method of replicating the working surface ofthe mold to form a negative copy of the plurality of cube cornerelements suitable for use as a mold for forming retroreflectivearticles, and a mold formed therefrom. A retroreflective article can beformed from the mold forming the negative copy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a perspective view of a single lamina suitable for usein the disclosed methods.

[0025]FIG. 2 is a perspective view of a plurality of such laminae.

[0026]FIG. 3 is an end view of the plurality of laminae oriented in afirst orientation.

[0027]FIG. 4 is an end view of the plurality of laminae following afirst machining operation.

[0028]FIG. 5 is a side view of the plurality of laminae following afirst machining operation.

[0029]FIG. 6 is an end view of the plurality of laminae depicted in FIG.5 oriented in a second orientation.

[0030]FIG. 7 is an end view of the plurality of laminae oriented in asecond orientation, wherein alternating lamina have been rotated 180°.

[0031]FIG. 8 is an end view of the plurality of laminae following asecond machining operation.

[0032]FIG. 9 is a top view of the plurality of laminae following asecond machining operation.

[0033]FIG. 10 is an end view of the plurality of laminae oriented in afirst orientation.

[0034]FIG. 11 is an end view of the plurality of laminae following afirst machining operation.

[0035]FIG. 12 is a side view of the plurality of laminae following afirst machining operation.

[0036]FIG. 13 is an end view of the plurality of laminae oriented in asecond orientation.

[0037]FIG. 14 is an end view of the plurality of laminae following asecond machining operation.

[0038]FIG. 15 is a side view of the plurality of laminae following asecond machining operation.

[0039]FIG. 16 is an end view of the plurality of laminae following athird machining operation.

[0040]FIG. 17 is a top view of the plurality of laminae following athird machining operation.

[0041]FIG. 18 is a perspective view of a single lamina according to themethod of FIGS. 10-17.

[0042]FIG. 19 is an end view of the plurality of laminae oriented in afirst orientation.

[0043]FIG. 20 is an end view of the plurality of laminae following afirst machining operation.

[0044]FIG. 21 is a side view of the plurality of laminae following afirst machining operation.

[0045]FIG. 22 is an end view of the plurality of laminae oriented in asecond orientation.

[0046]FIG. 23 is an end view of the plurality of laminae following asecond machining operation.

[0047]FIG. 24 is a side view of the plurality of laminae following asecond machining operation.

[0048]FIG. 25 is a side view of the plurality of laminae following athird machining operation.

[0049]FIG. 26 is a top view of the plurality of laminae following athird machining operation.

[0050]FIG. 27 is a perspective view of a single lamina according to themethod of FIGS. 19-26.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] A plurality of laminae are machined simultaneously to form aplurality of full cube corner elements. The three mutually perpendicularoptical faces of each cube corner element are preferably formed on asingle lamina. All three optical faces are preferably formed by themachining process to ensure optical quality surfaces. A planar interfaceis preferably maintained between adjacent laminae during the machiningphase and subsequent thereto so as to minimize alignment problems anddamage due to handling of the laminae.

[0052] In describing the various embodiments, specific terminology willbe used for the sake of clarity. Such terminology is not, however,intended to be limiting and it is to be understood that each term soselected includes all technical equivalents that function similarly. Thedisclosed methods can be used to form retroreflective elements of avariety of sizes and shapes, such as fall cube corner elements andtruncated cube corner elements. The base edges of adjacent truncatedcube corner elements in an array are typically coplanar. The base edgesof adjacent full cube corner elements in an array are not in the sameplane. Related applications filed on the same date herewith include:Cube Corner Sheeting Mold and Method Making the Same (Atty. Docket No.51946USA9A); Retroreflective Cube Corner Sheeting Mold and SheetingFormed Therefrom (Atty. Docket No. 53305USA5A); Retroreflective CubeCorner Sheeting, Molds Therefore, and Methods of Making the Same (Atty.Docket No. 53318USA8A); Tiled Retroreflective Sheeting Composed ofHighly Canted Cube Corner Elements (Atty. Docket No. 53285USA9A); DualOrientation Retroreflective Sheeting (Atty. Docket No. 52303USA8B).

[0053] For purposes of description, a Cartesian coordinate system can besuperimposed onto lamina 10. A first reference plane 24 is centeredbetween first major surface 12 and second major surface 14. Firstreference plane 24, referred to as the x-z plane, has the y-axis as itsnormal vector. A second reference plane 26, referred to as the x-yplane, extends substantially co-planar with working surface 16 of lamina10 and has the z-axis as its normal vector. A third reference plane 28,referred to as the y-z plane, is centered between first end surface 20and second end surface 22 and has the x-axis as its normal vector.Although various geometric attributes will be described herein withreference to such Cartesian reference planes, it will be appreciatedthat they can be described using other coordinate systems or withreference to the structure of the lamina.

[0054] One embodiment of a lamina, as well as a method of making thesame, will now be described with reference to FIGS. 1-9. In FIG. 1, arepresentative lamina 10 useful in the manufacture of a mold suitablefor forming retroreflective sheeting includes a first major surface 12and an opposing second major surface 14. Lamina 10 further includes aworking surface 16 and an opposing bottom surface 18 extending betweenfirst major surface 12 and second major surface 14. Lamina 10 furtherincludes a first end surface 20 and an opposing second end surface 22.In a one embodiment, lamina 10 can be a right rectangular polyhedronwherein opposing surfaces are substantially parallel. However, it willbe appreciated that opposing surfaces of lamina 10 need not be parallel.

[0055] FIGS. 2-9 illustrate one embodiment of the formation of astructured surface comprising a plurality of optically opposing cubecorner elements in the working surface 16 of lamina 10. In brief, theplurality of laminae 10 are oriented such that their respective firstreference planes 24 are disposed at a first angle θ₁, relative to afixed reference axis (FIG. 3). A first groove set comprising a pluralityof parallel, adjacent grooves 30a, 30b, 30c, etc. (collectively referredto by the reference numeral 30) is formed in the working surface 16 ofthe plurality of laminae 10 (FIGS. 3-5). The grooves of the first grooveset 30 define respective first groove surfaces 32a, 32b, 32c, etc. andrespective second groove surfaces 34b, 34c, 34d, etc. Importantly, therespective first groove surfaces 32a, 32b, 32c, etc. intersect therespective second groove surfaces 34b, 34c, etc. substantiallyorthogonally to define respective first reference edges 36a, 36b, 36c,etc. As used herein, the terms ‘substantially orthogonally’ or‘approximately orthogonally’ shall mean that the dihedral angle betweenthe respective surfaces measures approximately 90°; slight variations inorthogonality as disclosed and claimed in U.S. Pat. No. 4,775,219 toAppeldorn are contemplated by the present invention. A second groove setcomprising a plurality of parallel adjacent grooves 46a, 46b, 46c, etc.is also formed in the working surface 16 of lamina 10 (FIGS. 6-8). Thegrooves 46 divides and/or bisects the first and second groove surfaces32, 34. For the sake of clarity, groove surfaces on one side of thegroove 46 are referred to as the first and second groove surfaces 32, 34and the groove surfaces on the other side of the groove 46 are referredto as the third and forth groove surfaces 40, 42.

[0056] The grooves of the second groove set define respective fifthgroove surfaces 48a, 48b, 48c, etc. (collectively referred to by thereference numeral 48) and sixth groove surfaces 50a, 50b, 50c, etc.(collectively referred to by the reference numeral 50). The fifth groovesurfaces 48a, 48b, 48c, etc. intersect the respective first groovesurfaces 32a, 32b, 32c, etc. and second groove surfaces 34b, 34c, etc.substantially orthogonally to form a plurality of cube corner elements60a, 60b, 60c on the working surfaces 16 of the respective laminae.Similarly, the sixth groove surfaces 50a, 50b, 50c, etc. intersect therespective first groove surfaces 40a, 40b, 40c, etc. and second groovesurfaces 42b, 42c, etc. substantially orthogonally to form a pluralityof cube corner elements 70a, 70b, etc. on the working surfaces 16 of therespective laminae. As used herein, the term ‘groove set’ refers to allparallel grooves formed in working surface 16 of the laminae 10.

[0057] The embodiment will now be explained in greater detail. Turningback to FIG. 2, a plurality of thin laminae 10 are assembled togethersuch that the first major surface 12 of one lamina 10 is adjacent thesecond major surface 14 of an adjacent lamina 10. Preferably, theplurality of laminae 10 are assembled in a fixture of conventionaldesign capable of securing the plurality of laminae adjacent oneanother. The fixture preferably defines a base plane 80 (FIG. 3) whichis preferably substantially parallel to the bottom surfaces 18 of thelaminae 10 when the laminae 10 are positioned as shown in FIG. 2. Theplurality of laminae 10 can be characterized by a Cartesian coordinatesystem as described above. Preferably, working surfaces 16 of theplurality of laminae 10 are substantially coplanar when the laminae arepositioned with their first reference planes 24 perpendicular to baseplane 80.

[0058] In FIG. 3, the plurality of laminae 10 are oriented to have theirfirst reference planes 24 disposed at a first angle θ₁ from a fixedreference axis 82 normal to base plane 80. In one embodiment, the firstangle θ₁ is approximately 27.80°. However, in practice θ₁ can be betweenabout 1° and about 85°, and more preferably between about 10° and about60°, and most preferably between about 25° and about 45°.

[0059] Referring to FIGS. 4-5, a first groove set comprising a pluralityof parallel adjacent V-shaped grooves 30a, 30b, 30c, etc. (collectivelyreferred to by reference numeral 30) is formed in the working surfaces16 of the plurality of laminae 10 with the lamina disposed at angle θ₁.At least two such grooves 30 are formed in working surface 16 of theplurality of laminae 10. The grooves 30 define first groove surfaces32a, 32b, 32c, etc. (collectively referred to by reference numeral 32)and second groove surfaces 34b, 34c, 34d, etc. (collectively referred toby reference numeral 34) that intersect as shown at groove vertices 33b,33c, 33d, etc. (collectively referred to by the reference numeral 33).At the edge of the laminae, the groove forming operation may form asingle groove surface 32a. Groove surfaces 32a and 34b of adjacentgrooves 30a, 30b intersect approximately orthogonally along a referenceedge 36a. Similarly, adjacent groove surfaces 32b and 34c intersectapproximately orthogonally along reference edge 36b. This can beaccomplished by forming grooves 30 using a cutting tool having a 90°included angle. Preferably this pattern is repeated across the entireworking surfaces 16 of the plurality of laminae 10. Groove vertices 33are preferably spaced apart by between about 0.01 millimeters and about1.0 millimeters, however these values are not intended to be limiting.

[0060] Grooves 30 are formed by removing portions of working surface 16of the plurality of laminae using suitable material removal techniquesincluding precision machining techniques such as milling, ruling,grooving and fly-cutting. Chemical etching or laser ablation techniquescan also be used. In one embodiment, grooves 30 are formed in ahigh-precision machining operation in which a diamond cutting toolhaving a 90° included angle is repeatedly moved transversely across theworking surfaces 16 of the plurality of laminae 10 along an axis that issubstantially parallel to base plane 80. The diamond cutting tool could,however, be moved along an axis that is non-parallel to base plane 80such that the tool cuts at a varying depth across the plurality oflaminae 10. Further, the machining tool can be held stationary while theplurality of laminae are placed in motion; any relative motion betweenlaminae 10 and the machining tool is contemplated.

[0061] In the embodiment of FIGS. 2-5 , the grooves 30 of the firstgroove set are formed at a depth such that the respective firstreference edges 36 intersect the first major surface 12 and the secondmajor surface 14 of each lamina. Thus, in the end view depicted in FIG.4, the reference edges 36 and groove vertices 33 form substantiallycontinuous lines that extend along an axis parallel to base plane 80.Further, grooves 30 are formed such that the respective reference edges36 are disposed in a plane that intersects the respective firstreference planes 24 and the second reference plane 26 at orthogonalangles. Thus, in a top plan view the respective first reference edges 36would appear perpendicular to the respective first reference planes 24of the plurality of laminae 10. However, grooves 30 can also have lesserdepths. For example, if the depth of the tool is decreased, the groovevertices 33 will be formed closer to the working surface 16 and flat,transmissive regions will be formed.

[0062] To complete the formation of cube corner elements on the workingsurfaces 16 of the laminae 10, a second groove set is formed bymachining a single groove in each lamina 10 along an axis substantiallyparallel with first reference plane 24. In the embodiment illustrated inFIGS. 6-8, the plurality of lamina 10 are removed from the assembly andalternating laminae (10b, 10d, etc.) are rotated 180° about an axisperpendicular to second reference plane 26. The plurality of laminae arethen reassembled with their respective first reference planes 24preferably disposed substantially perpendicular to base plane 80 asdepicted in FIG. 7.

[0063] Referring to FIGS. 8 and 9, a second groove set that preferablyincludes at least one groove 46 in each lamina 10 is formed in theworking surface 16 of the plurality of laminae 10. In the disclosedembodiment the second grooves 46a, 46b, 46c, etc. (collectively referredto as 46) define respective fifth groove surfaces 48a, 48b, 48c, etc.(collectively referred to as 48) and sixth groove surfaces 50a, 50b,50c, etc. (collectively referred to as 50) that intersect at respectivegroove vertices 52a, 52b, 52c, etc. (collectively referred to as 52)along axes that are perpendicular to the third reference plane 28.

[0064] The second grooves 46 are formed such that fifth groove surfaces48 are substantially orthogonal to the respective first groove surfaces(e.g. 32a, 32b, etc.) and second groove surfaces (e.g. 34a, 34b, etc.).Formation of the fifth groove surfaces 48 as described yields aplurality of cube corner elements 60a, 60b, etc. (collectively referredto as 60) in working surface 16 of alternating laminae 10. Each cubecorner element 60 is defined by a first groove surface (32a, 32b, etc.),a second groove surface (34a, 34b, etc.) and a portion of a fifth groovesurface 48 that mutually intersect at a point to define a cube cornerpeak, or apex 62. Similarly, the sixth groove surfaces 50 aresubstantially orthogonal to the respective third groove surfaces (e.g.40a, 40b, etc.) and fourth groove surfaces (e.g. 42a, 42b, etc.). Asnoted above, third and fourth groove surfaces 40, 42 were formed by thefirst groove set 30. Formation of the sixth groove surface 50 alsoyields a plurality of cube corner elements 70a, 70b, etc. (collectivelyreferred to as 70) in working surface 16 of alternating laminae 10. Eachcube corner element 70 is defined by a third groove surface (40a, 40b,etc.), a fourth groove surface (42a, 42b, etc.) and a portion of sixthgroove surface 50 that mutually intersect at a point to define a cubecorner peak, or apex 72. Preferably, both groove surfaces 48 and 50 forma plurality of cube corner elements on the working surface 16 of lamina10. However, it will be appreciated that the second groove 46 can beformed such that only groove surface 48 or groove surface 50 forms cubecorner elements.

[0065] The cube corner elements 60, 70 are opposing pairs that generateopposing, although not necessarily identical, retroreflection patterns.The cube corner elements 60, 70 preferably generate symmetrical ormirror image retroreflection patterns, such as elements that aresubstantially identical but are rotated 180° relative to each other. Inan alternate embodiment, the second groove set 46 can be cut in thestack of laminae shown in FIG. 6 so that the resulting cube cornerelements 60, 70 are all aligned in the same direction. That is, thesymmetry axes or optical axes of the cube corner elements 60, 70 aregenerally parallel. Similarly, the laminae 10b, 10d, etc. can be rotated180° after the second groove set 46 is cut (see FIG. 8). The total lightreturn for cube corner elements 60, 70 aligned in the same direction isasymmetrical about a 360° range of orientation angles. An asymmetricalretroreflection pattern can be desirable for some applications, such aspavement markers or other items that are viewed from a narrow range oforientation angles.

[0066] A method of the present disclosure involves simultaneouslymachining a plurality of laminae, each lamina comprising one or morediscrete cube corner elements. The cube corner elements preferably donot extend across more than one lamina. For example, the three mutuallyperpendicular optical faces 32, 34, 48 of cube corner elements 60 aremachined on a single lamina. Similarly, the three optical faces 40, 42,50 of the cube corner elements 70 are machined on a single lamina. Thecube corner elements 60, 70 can be located on the same or differentlaminae. The cube corner elements 60, 70 are advantageously formed withonly two groove sets 30, 46 by the machining process to ensure anoptical quality surface. A planar interface between major surfaces 12,14 is maintained between adjacent laminae during the machining phase andin the subsequent mold formed therefrom so as to minimize alignmentproblems and damage due to handling of the laminae, to minimize gapsbetween adjacent laminae that would degrade the quality of negativecopies, and to minimize flash from migrating into the gaps between thelaminae.

[0067] FIGS. 10-18 illustrate an alternate method of forming the mold ofFIGS. 1-9 on a plurality of laminae as illustrated in FIG. 2, usingthree groove sets 130, 138, 146. Preferably, the respective workingsurfaces 116 of the plurality of laminae 110 are substantially coplanarwhen the lamina are positioned with their respective first referenceplanes 124 perpendicular to base plane 180. The reference planes 124,126, 128 correspond to the reference planes 24, 26, 28, respectively,discussed above.

[0068] Referring to FIG. 10, the plurality of laminae 110 are orientedto have their first reference planes 124 disposed at a first angle β₁,from a fixed reference axis 182 normal to base plane 180. In oneembodiment, β₁ is approximately 27.8°. However, β₁ can alternately bebetween about 1° and about 85°, and more preferably between about 10°and about 60°.

[0069] Referring to FIGS. 11-12, a first groove set comprising aplurality of parallel adjacent V-shaped grooves 130a, 130b, 130c, etc.(collectively referred to as 130) is formed in the working surfaces 116of the plurality of laminae 110 with the lamina disposed at angle β₁. Atleast two such grooves 130 are formed in working surface 116 of theplurality of laminae 110. The grooves 130 define first groove surfaces132a, 132b, 132c, etc. (collectively referred to as 132) and secondgroove surfaces 134b, 134c, 134d, etc. (collectively referred to as 134)that intersect as shown at groove vertices 133b, 133c, 133d, etc.(collectively referred to as 133). At the edge of the lamina, the grooveforming operation can form a single groove surface 132a. Groove surfaces132a and 134b of adjacent grooves intersect approximately orthogonallyalong a reference edge 136a. Similarly, adjacent groove surfaces 132band 134c intersect approximately orthogonally along reference edge 136b.Preferably this pattern is repeated across the entire working surfaces116 of the plurality of laminae 110.

[0070] Grooves 130 are formed by removing portions of working surface116 of the plurality of laminae using suitable material removaltechniques including precision machining techniques such as milling,ruling, grooving and fly-cutting. Chemical etching or laser ablationtechniques can also be used. In one embodiment, the grooves 130 areformed in a high-precision machining operation in which a diamondcutting tool having a 90° included angle is repeatedly movedtransversely across the working surfaces 116 of the plurality of laminae110 along an axis that is substantially parallel to base plane 180. Thediamond cutting tool could, however, alternately be moved along an axisthat is non-parallel to base plane 180 such that the tool cuts at avarying depth across the plurality of laminae 110. Further, themachining tool could be held stationary while the plurality of laminaeare placed in motion; any relative motion between the laminae 110 andthe machining tool is contemplated.

[0071] In the embodiment of FIGS. 11-12, the grooves 130 are formed at adepth such that the respective first reference edges 136 intersect thefirst major surface 112 and the second major surface 114 of each lamina.Thus, in the end view of FIG. 11, the reference edges 136 and groovevertices 133 form substantially continuous lines that extend along anaxis parallel to base plane 180. Further, grooves 130 are formed suchthat the respective reference edges 136 are disposed in a plane thatintersects the respective first reference planes 124 and the secondreference plane 126 at orthogonal angles. Thus, the respective firstreference edges 136 would appear perpendicular to the respective firstreference planes 124 of the plurality of laminae 110. However, grooves130 can also have lesser depths so as to form flat transmissive regions.

[0072] Referring to FIG. 13, the plurality of laminae 110 are thenoriented to have their respective first reference planes 124 disposed ata second angle β₂, from fixed reference axis 182 normal to base plane180. In one embodiment, β₂ is approximately 27.8°. However, in practiceβ₂ can be between about 1° and about 85°, but preferably between about10° and about 60°. The angle β₂ is independent of angle β₁ and need notequal β₁. To orient the plurality of laminae 110 at angle β₂, thelaminae 110 are preferably removed from the fixture and reassembled withtheir respective first reference planes disposed at angle β₂.

[0073] Referring to FIGS. 14-15, a second groove set comprising aplurality of parallel adjacent V-shaped grooves 138b, 138c, etc.(collectively referred to as 138) is formed in the working surfaces 116of the plurality of laminae 110 with the lamina disposed at angle β₂. Atleast two adjacent grooves 138 are formed in working surface 116 of theplurality of laminae 110. The grooves 138 define third groove surfaces140a, 140b, 140c, etc. (collectively referred to as 140) and fourthgroove surfaces 142b, 142c, 142d, etc. (collectively referred to as 142)that intersect as shown at groove vertices 141b, 141c, 141d, etc.(collectively referred to as 141). At the edge of the lamina, the grooveforming operation can form a single groove surface 140a. Groove surfaces140a and 142b of adjacent grooves intersect approximately orthogonallyalong a reference edge 144a. Similarly, adjacent groove surfaces 140band 142c intersect approximately orthogonally along reference edge 144b.Preferably this pattern is repeated across the entire working surfaces116 of the plurality of laminae 110.

[0074] Grooves 138 of the second groove set are also preferably formedby a high-precision machining operation in which a diamond cutting toolhaving a 90° included angle is repeatedly moved transversely across theworking surfaces 116 of the plurality of laminae 110 along a cuttingaxis that is substantially parallel to base plane 180. Again, it will benoted that it is important that the surfaces of adjacent grooves 138intersect along the reference edges 144 to form orthogonal dihedralangles. The included angle of each groove can measure other than 90°.Grooves 138 are preferably formed at approximately the same depth inworking surface 116 of the plurality of laminae 110 as grooves 130 infirst groove set. Additionally, the grooves 138 in the second groove setare preferably formed such that the respective groove vertices (e.g.141a, 141b, etc.) and the respective reference edges (e.g. 144a, 144b,etc.) are substantially coplanar with respective groove vertices (e.g.133a, 133b, etc.) and the respective reference edges (e.g. 136a, 136b,etc.) of the grooves 130 in the first groove set.

[0075] Referring to FIGS. 16-17, a third groove set that preferablyincludes at least one groove 146 in each lamina 110 is formed in theworking surface 116 of the plurality of laminae 110. In the disclosedembodiment the third grooves 146a, 146b, 146c, etc. (collectivelyreferred to as 146) define respective fifth groove surfaces 148a, 148b,148c, etc. (collectively referred to as 148) and respective sixth groovesurfaces 150a, 150b, 150c, etc. (collectively referred to as 150) thatintersect at respective groove vertices 152a, 152b, 152c, etc.(collectively referred to as 152) along axes that are parallel to therespective first reference planes 124. The third grooves 146 are formedsuch that respective fifth groove surfaces 148 are disposed in a planethat is substantially orthogonal to the respective first groove surfaces(e.g. 132a, 132b, etc.) and the respective second groove surfaces (e.g.134a, 134b, etc.). Formation of the fifth groove surfaces 148 in thisway yields a plurality of cube corner elements 160a, 160b, etc.(collectively referred to as 160) in working surface 116 of therespective lamina 110.

[0076] Each cube corner element 160 is defined by a first groove surface(132a, 132b, etc.), a second groove surface (134b, 134c, etc.) and aportion of a fifth groove surface 148 that mutually intersect at a pointto define a cube corner peak, or apex 162. Similarly, sixth groovesurface 150 is disposed in a plane that is substantially orthogonal tothe respective third groove surfaces (e.g. 140a, 140b, etc.) and therespective fourth groove surfaces (e.g. 142b, 142c, etc.). Formation ofthe sixth groove surface 150 also yields a plurality of cube cornerelements 170a, 170b, etc. (collectively referred to as 170) in workingsurface 116 of lamina 110. Each cube corner element 170 is defined by athird groove surface (140a, 140b, etc.), a fourth groove surface (142a,142b, etc.) and a portion of sixth groove surface 150 that mutuallyintersect at a point to define a cube corner peak, or apex 172.Preferably, both fifth groove surface 148 and sixth groove surface 150form a plurality of cube corner elements on the working surface 116 oflamina 110. However, it will be appreciated that third groove 146 can beformed such that only fifth groove surface 148 or sixth groove surface150 forms cube corner elements.

[0077] In a preferred method the plurality of laminae 110 arere-oriented to have their respective first reference planes 124 disposedapproximately parallel to reference axis 182 before forming theplurality of grooves 146. However, the grooves 146 can be formed withthe lamina oriented such that their respective first reference planesare disposed at an angle relative to reference axis 182. In particular,in some embodiments it may be advantageous to form the respective thirdgrooves 146 with the respective lamina 110 disposed at angle β₂ to avoidan additional orientation step in the manufacturing process. Preferably,grooves 146 are also formed by a high precision machining operation. Inthe disclosed embodiment a diamond cutting tool having an included angleof about 55.6° is moved across the working surface 116 of each lamina110 along an axis that is substantially contained by the first referenceplane 124 of the lamina 110 and that is parallel to base plane 180.Grooves 146 are preferably formed such that the respective groovevertices 152 are slightly deeper than the vertices of the grooves in thefirst and second groove sets. Formation of grooves 146 result in aplurality of laminae 110 having a structured surface substantially asdepicted in FIG. 18.

[0078] As discussed in connection with FIGS. 1-9, the method of FIGS.10-18 results in simultaneously machining a plurality of laminae, eachhaving cube corner elements 160 with three mutually perpendicularoptical faces 132, 134, 148 on a single lamina. Similarly, the threeoptical faces 140, 142, 150 of the cube corner elements 170 are machinedon a single lamina. A planar interface between major surfaces 112, 114is maintained between adjacent laminae during the machining phase and inthe subsequent mold formed therefrom so as to minimize alignmentproblems and damage due to handling of the laminae.

[0079] FIGS. 19-27 illustrate an alternate embodiment of simultaneouslyforming a plurality of cube corner elements on a plurality of laminae,such as illustrated in FIG. 2. Preferably, the respective workingsurfaces 216 of the laminae 210 are substantially coplanar when thelamina are positioned with their respective first reference planes 224perpendicular to base plane 280. The reference planes 224, 226, 228correspond to the reference planes 24, 26, 28, respectively, discussedabove.

[0080] Referring to FIG. 19, the plurality of laminae 210 are orientedto have their first reference planes 224 disposed at a first angle θ₁,from a fixed reference axis 282 normal to base plane 280. In oneembodiment, θ₁ is approximately 54.74°. In theory, θ₁ can be any anglebetween about 45° and about 90°, however, in practice it is typicallybetween approximately about 45° and about 60°. Referring to FIGS. 20-21,a first groove set comprising a plurality of parallel adjacent V-shapedgrooves 230a, 230b, 230c, etc. (collectively referred to as 230) isformed in the working surfaces 216 of the plurality of laminae 210 withthe lamina disposed at angle θ₁. The grooves 230 define first groovesurfaces 232a, 232b, 232c, etc. (collectively referred to as 232) andsecond groove surfaces 234b, 234c, 234d, etc. (collectively referred toas 234) that intersect at groove vertices 233b, 233c, 233d, etc.(collectively referred to by the reference numeral 233) as shown. At theedge of the lamina, the groove forming operation can form a singlegroove surface, e.g. 232a, 234d. Preferably this pattern is repeatedacross the entire working surfaces 216 of the plurality of laminae 210.

[0081] Grooves 230 are formed by removing portions of working surface216, as discussed above. In one embodiment, the grooves 230 are formedin a high-precision machining operation in which a diamond cutting toolhaving a 120° included angle repeatedly moves transversely across theworking surfaces 216 of the plurality of laminae 210 along an axis thatis substantially parallel to base plane 280. It will be appreciated,however that the diamond cutting tool can move along an axis that isnon-parallel to base plane 280 such that the tool cuts at a varyingdepth across the plurality of laminae 210.

[0082] In the embodiment of FIGS. 20-21, the grooves 230 are formed at adepth such that the respective groove vertices 233 intersect the firstmajor surface 212 and the second major surface 214 of each lamina. Thus,in the end view depicted in FIG. 20, groove vertices 233 formsubstantially continuous lines that extend along an axis parallel tobase plane 280. Further, grooves 230 are formed such that the groovevertices 233 and the edges 236 are disposed in planes that intersect thefirst reference planes 224 and the second reference planes 226 atorthogonal angles. The respective groove vertices appear perpendicularto the respective first reference planes 224 of the plurality of laminae210. However, grooves 230 can be formed at lesser depths or alongdifferent axes.

[0083] Referring to FIGS. 22-23, the plurality of laminae 210 are thenoriented to have their respective first reference planes 224 disposed ata second angle θ₂, from fixed reference axis 282 normal to base plane280 and a second groove set comprising a plurality of parallel adjacentV-shaped grooves 238a, 238b, 238c, etc. (collectively referred to as238) is formed in the working surfaces 216 of the plurality of laminae210. In the disclosed embodiment, θ₂ is approximately 54.74°. Asdiscussed above, in theory, θ₂ can be any angle between about 45° andabout 90°, however, in practice it is preferably between about 45° andabout 60°. To orient the plurality of laminae 210 at angle θ₂, thelaminae 210 are preferably removed from the fixture and reassembled withtheir respective first reference planes disposed at angle θ₂. Thegrooves 238 define third groove surfaces 240a, 240b, 240c, etc.(collectively referred to as 240) and fourth groove surfaces 242b, 242c,242d, etc. (collectively referred to as 242) that intersect at groovevertices 241b, 241c, 241d, etc. (collectively referred to as 241) andalong edges 247a, 247b, 247c, etc. as shown. At the edge of the lamina,the groove forming operation can form a single groove surface.Preferably this pattern is repeated across the entire working surfaces216 of the laminae 210.

[0084] Grooves 238 of the second groove set are also preferably formedby a high-precision machining operation in which a diamond cutting toolhaving an included angle of about 120° repeatedly moves transverselyacross the working surfaces 216 of the laminae 210 along a cutting axissubstantially parallel to base plane 280. Grooves 238 are preferablyformed at approximately the same depth as grooves 230. Additionally,grooves 238 are preferably formed such that the groove vertices (e.g.241a, 241b, etc. ) are substantially coplanar with respective groovevertices (e.g. 233a, 233b, etc.) of the grooves 230. After forming thegrooves 238 in the second groove set, each lamina 210 preferably appearsas shown in FIG. 27.

[0085] Referring to FIG. 25-26, a third groove set comprising aplurality of parallel adjacent V-shaped grooves 246a, 246b, 246c etc.(collectively referred to as 246) is formed in the working surfaces 216of the plurality of laminae 210. The third grooves 246 define fifthgroove surfaces 248a, 248b, 248c, etc. (collectively referred to as 248)and respective sixth groove surfaces 250a, 250b, 250c, etc.(collectively referred to as 250) that intersect at groove vertices252a, 252b, 252c, etc. (collectively referred to as 252). The thirdgrooves 246 are formed such that the fifth groove surfaces 248 aredisposed substantially orthogonal to the respective first groovesurfaces 232 and the respective third groove surfaces 240.

[0086] Formation of the fifth groove surfaces 248 as described yields aplurality of cube corner elements (e.g. 260a, 260b, 260c, etc.),collectively referred to by reference numeral 260, in working surface216 of the respective lamina 210. Each cube corner element 260 isdefined by a first groove surface 232 a third groove surface 240 and afifth groove surface 248 that mutually intersect at a point to define acube corner peak, or apex 262. Similarly, the sixth groove surfaces 250are disposed substantially orthogonal to the respective second groovesurfaces 234 and the respective fourth groove surfaces 242. Formation ofthe sixth groove surfaces 250 also yields a plurality of cube cornerelements 270a, 270b, etc. (collectively referred to by reference numeral270) in working surface 216 of lamina 210. Each cube corner element 270is defined by a second groove surface 234, a fourth groove surface 242and a sixth groove surface 250 that mutually intersect at a point todefine a cube corner peak, or apex 272. Preferably, both fifth groovesurface 248 and sixth groove surface 250 form a plurality of opticallyopposing cube corner elements on the working surface 216 of lamina 210.However, it will be appreciated that third groove 246 could be formedsuch that only fifth groove surfaces 248 or sixth groove surfaces 250forms cube corner elements.

[0087] In a preferred method the plurality of laminae 210 arere-oriented to have their respective major planes 224 disposedapproximately parallel to reference axis 282 before forming theplurality of grooves 246. In a preferred embodiment a diamond cuttingtool having an included angle of 90° moves across the working surfaces216 of the plurality of laminae 210 along an axis that is substantiallyparallel to base plane 280. However, the grooves 246 can be formed withthe lamina oriented such that their respective major planes are disposedat an angle relative to reference axis 282. Grooves 246 are preferablyformed such that the respective groove vertices 252 are slightly deeperthan the vertices of the grooves in the first and second groove sets.Formation of grooves 246 result in a plurality of laminae 210 having astructured surface substantially as depicted in FIG. 27.

[0088] Working surface 216 exhibits several desirable characteristics asa retroreflective article. The cube corner element geometry formed inworking surface 216 of lamina 210 may be characterized as a ‘full’ or‘high efficiency’ cube corner element geometry because the geometryexhibits a maximum effective aperture that approaches 100%. Thus, aretroreflective article formed as a replica of working surface 216 willexhibit high optical efficiency in response to light incident on theretroreflective article approximately along the symmetry axes of thecube corner elements. Additionally, cube corner elements 260 and 270 canbe disposed in opposing orientations and are symmetrical with respect tofirst reference plane 24 and will exhibit symmetric retroreflectiveperformance in response to light incident on the retroreflective articleat high entrance angles.

[0089] The laminae are preferably formed from a dimensionally stablematerial capable of holding precision tolerances, e.g. machinableplastics (for example, polyethylene teraphthalate, polymethylmethacrylate, and polycarbonate) or metals (for example, brass, nickel,copper, or aluminum). The physical dimensions of the laminae areconstrained primarily by machining limitations. Each lamina preferablymeasures between about 0.025 millimeters and about 1.0 millimeters inthickness, and more preferably about 0.1 to about 0.6 millimeters,between about 5 and about 100 millimeters in height, and between about10 and about 500 millimeters in width. These measurements are providedfor illustrative purposes only and are not intended to be limiting.

[0090] In the manufacture of retroreflective articles such asretroreflective sheeting, the structured surface of the plurality oflaminae is used as a master mold which can be replicated usingelectroforming techniques or other conventional replicating technology.The plurality of laminae can include substantially identical cube cornerelements or may include cube corner elements of varying sizes,geometries, or orientations. The structured surface of the replica,referred to in the art as a ‘stamper’, contains a negative image of thecube corner elements. This replica can be used as a mold for forming aretroreflective article. More commonly, however, a large number ofpositive or negative replicas are assembled to form a mold large enoughto be useful in forming retroreflective sheeting. Retroreflectivesheeting can then be manufactured as an integral material, e.g. byembossing a preformed sheet with an array of cube corner elements asdescribed above or by casting a fluid material into a mold. See, JP8-309851 and U.S. Pat. No. 4,601,861 (Pricone). Alternatively, theretroreflective sheeting can be manufactured as a layered product bycasting the cube corner elements against a preformed film as taught inPCT application No. WO 95/11464 and U.S. Pat. No. 3,648,348 or bylaminating a preformed film to preformed cube corner elements. By way ofexample, such sheeting can be made using a nickel mold formed byelectrolytic deposition of nickel onto a master mold. The electroformedmold can be used as a stamper to emboss the pattern of the mold onto apolycarbonate film approximately 500 μm thick having an index ofrefraction of about 1.59. The mold can be used in a press with thepressing performed at a temperature of approximately 175° to about 200°C.

[0091] Useful materials for making such reflective sheeting arepreferably materials that are dimensionally stable, durable, weatherableand readily formable into the desired configuration. Examples ofsuitable materials include acrylics, which generally have an index ofrefraction of about 1.5, such as Plexiglas resin from Rohm and Haas;thermoset acrylates and epoxy acrylates, preferably radiation cured,polycarbonates, which have an index of refraction of about 1.6;polyethylene-based ionomers (marketed under the name ‘SURLYN’);polyesters; and cellulose acetate butyrates. Generally any opticallytransmissive material that is formable, typically under heat andpressure, can be used. Other suitable materials for formingretroreflective sheeting are disclosed in U.S. Pat. No. 5,450,235 toSmith et al. The sheeting can also include colorants, dyes, UVabsorbers, or other additives as needed.

[0092] It is desirable in some circumstances to provide retroreflectivesheeting with a backing layer. A backing layer is particularly usefulfor retroreflective sheeting that reflects light according to theprinciples of total internal reflection. A suitable backing layer can bemade of any transparent or opaque material, including colored materials,that can be effectively engaged with the disclosed retroreflectivesheeting. Suitable backing materials include aluminum sheeting,galvanized steel, polymeric materials such as polymethyl methacrylates,polyesters, polyamids, polyvinyl fluorides, polycarbonates, polyvinylchlorides, polyurethanes, and a wide variety of laminates made fromthese and other materials.

[0093] The backing layer or sheet can be sealed in a grid pattern or anyother configuration suitable to the reflecting elements. Sealing can beaffected by use of a number of methods including ultrasonic welding,adhesives, or by heat sealing at discrete locations on the arrays ofreflecting elements (see, e.g. U.S. Pat. No. 3,924,928). Sealing isdesirable to inhibit the entry of contaminants such as soil and/ormoisture and to preserve air spaces adjacent the reflecting surfaces ofthe cube corner elements.

[0094] If added strength or toughness is required in the composite,backing sheets of polycarbonate, polybutryate or fiber-reinforcedplastic can be used. Depending upon the degree of flexibility of theresulting retroreflective material, the material can be rolled or cutinto strips or other suitable designs. The retroreflective material canalso be backed with an adhesive and a release sheet to render it usefulfor application to any substrate without the added step of applying anadhesive or using other fastening means.

[0095] The cube corner elements disclosed herein can be individuallytailored so as to distribute light retroreflected by the articles into adesired pattern or divergence profile, as taught by U.S. Pat. No.4,775,219. Typically the groove half-angle error introduced will be lessthan ±20 arc minutes and often less than ±5 arc minutes.

[0096] For convenience, the working surfaces of a plurality of laminaewhen considered collectively can be referred to as a collective workingsurface. Thus, for example, grooves 30 of FIG. 5, having groove bottoms33 and reference edges 36 (see FIGS. 4 and 5), are formed in thecollective working surface of the plurality of laminae 10 (see FIGS.1-4) and also are formed in the working surface 16 of each such lamina10. As another example, grooves 46 of FIG. 8, having groove bottoms 52,are also formed in the collective working surface of the plurality oflaminae 10, but are not fully formed in the working surface 16 of eachsuch lamina 10.

[0097] All patents and patent applications referred to, including thosedisclosed in the background of the invention, are hereby incorporated byreference. The present invention has now been described with referenceto several embodiments thereof. It will be apparent to those skilled inthe art that many changes can be made in the embodiments describedwithout departing from the scope of the invention. Thus, the scope ofthe present invention should not be limited to the preferred structuresand methods described herein, but rather by the broad scope of theclaims which follow.

What is claimed is:
 1. An article comprising an array of nonrulable cubecorner elements, the article comprising a plurality of laminae, eachsuch lamina having opposed parallel major surfaces and a working surfaceconnecting the major surfaces, the array having rows of rectangular cubecorner elements formed along the working surfaces of the laminae.
 2. Anarticle comprising a plurality of laminae, each lamina having opposedparallel major surfaces and a working surface connecting the majorsurfaces, the working surface of each lamina having an inclined surfaceextending along the working surface and a set of parallel groovesdefining groove surfaces orthogonal to each other and to the inclinedsurface so as to form a row of rectangular cube corner elements.
 3. Alamina having opposed parallel major surfaces and a working surfacetherebetween, the working surface having an inclined surface extendingtherealong and a set of parallel grooves defining groove surfacesorthogonal to each other and to the inclined surface so as to form a rowof rectangular cube corner elements.
 4. A plurality of laminae as setforth in claim 3 , the plurality of laminae defining in the workingsurfaces thereof a nonrulable array of cube corner elements.